The present invention relates to a magnet roll for use in an image forming apparatus utilizing electrophotography or electrostatic recording, such as a copying machine, a printer, etc., and a developing method using the magnet roll.
The image forming apparatus conventionally used includes a copying machine, a printer, a facsimile machine, etc. In the image forming machine utilizing an electrophotographic technique or an electrostatic recording technique, the image is produced by successive steps comprising (1) forming an electrostatic latent image corresponding to the information being reproduced on a surface of a photosensitive drum (image-bearing member) by exposing the uniformly charged photosensitive surface to image light; (2) magnetically attracting a magnetic developer (one-component developer comprising a magnetic toner, or two-component developer comprising a toner and a magnetic carrier) on a rotating developer feed roll disposed closely opposing the photosensitive drum, and (3) transporting the magnetic developer to a developing zone where the electrostatic latent image on the photoconductive drum attract toner from the magnetic toner to produce toner images.
The developer feed means mainly comprises a hollow cylindrical sleeve made of non-magnetic material, a permanent magnet roll interiorly mounted in the sleeve to effectively retain the magnetic developer on the sleeve surface and transport the magnetic developer to a developing zone, a shaft supporting the rotating sleeve or permanent magnet, and a doctor blade to regulating the magnetic developer layer on the sleeve surface within a suitable thickness. The developer feed means is partially received in a housing for storing the magnetic developer, and the permanent magnet roll is located opposite to the photosensitive drum with a constant, small spacing (developing gap).
Under the recently increasing demand for improving the image quality, supplying an image forming apparatus of low cost, and reducing the size of the image forming apparatus, various proposals have been made on the developing unit which is a major part of the image forming apparatus. Of such proposals, there has been proposed a sleeve-less development where a magnetic developer is directly retained on a surface of a rotating magnet roll having no sleeve (sleeve-less magnet roll) and transported to a developing zone. For example, JP-B-54-39139 discloses a developing unit where a surface of a sleeve-less magnet roll having a electroconductive rotating shaft is covered with an electroconductive layer and a developing current is allowed to flow between an photosensitive surface and the electroconductive layer via electroconductive toners by electrically interconnecting the electroconductive layer and the rotating shaft and grounding the photosensitive surface and the rotating shaft or connecting them to a bias source. JP-A-62-201463 discloses a developing method where a developer is transported to a developing zone by retaining the developer on a rotating sleeve-less magnet roll having an electroconductive surface and a plurality of magnetic poles on the surface, and the development is conducted while applying a bias voltage including zero bias between an image-bearing member and the magnet roll.
In the conventional sleeve-less development mentioned above, the developer is necessary to be biased in the developing zone, particularly in the reverse development, to attain a high image quality with less background fogging. Therefore, it has been proposed to coat the sleeve-less magnet roll with an electroconductive layer such as a plating layer, or make the entire body of the magnet roll electrically conductive by producing a resin-boned magnet from a material blended with an electroconductive powder. However, the proposed technique of making a plating layer involves several problems. For example, since a sintered magnet such as a ferrite sintered magnet is porous with a great number of voids, a plating solution penetrates into the magnet roll when the magnet roll is subjected to electroplating or electroless plating. The penetrated solution is difficult to be thoroughly removed from the magnet body to cause corrosion of the magnet roll. Also, the binding strength between the surface of a permanent magnet and the plating layer is usually insufficient. Further, the addition of the electroconductive powder to the material for the resin-bonded magnet reduces the magnetic force of the resultant resin-bonded magnet because the content of the ferromagnetic powder is reduced according to the addition amount of the electroconductive powder.